Digital display devices, such as liquid crystal display (LCD) monitors, typically receive analog signals from a video source (e.g., a personal computer) and convert the analog signals into a digital image. This conversion process involves various processing steps including an analog-to-digital conversion. The analog-to-digital conversion process is typically performed by a digitizer, such as an analog-to-digital (A/D) converter. The A/D converter receives a system clock signal that controls the operation of the converter. Particularly, the pulses of the clock signal are used to sample the rising and/or falling edges of the analog input signal. The conversion process requires the sampling frequency and phase of the clock signal to be precisely synchronized with the analog input signal. That is, to properly digitize the analog input signal, a line locked A/D converter clock signal, which is locked to the input signals at both frequency and phase, is required. Both the frequency lock and the phase lock are important to the conversion process. If the frequency is not properly locked to the input, some lower frequency components may be generated. If the phase is not correct, the A/D converter may sample the transition of the input analog signal, which causes serious noise at the digitized result. This will cause the display image to be degraded overall, include areas that are blurred, or be misaligned to the display area of the display monitor.
There are many methods to determine the phase for the sampling clock signal, such as spatial or temporal methods. Both spatial and temporal methods require the input signal to have several static areas with substantial transitions between the static areas. For a PC video input (e.g., PC_RGB), phase detection is relatively easy, since most PC content is static graphics with substantial transitions. But for a High Density Television (HDTV) input (e.g., HD_YPbPr), phase detection is more difficult, since most of the video content is motion picture graphics, which are not as sharp as PC graphics.
Therefore, it would be desirable to provide a system and method for automatically controlling the phase of a clock signal for sampling an HDTV signal that uses a new and improved method for phase detection.